Drive shaft for acoustic imaging catheters and flexible catheters

ABSTRACT

A shaft is shown having inner and outer members in interfering contact along their length, at least one of which comprises a wire, at least a portion of which is superelastic, to achieve substantial mechanical fidelity and resistance to damage. An ultrasound imaging catheter has a hollow catheter shaft, an ultrasound transducer in the shaft, and a drive shaft that rotates the transducer. The shaft comprises at least one metal coil having characteristic elastic deformation under stress in the range of about 3% to 9%, while a restraint means prevents torsional deflection of the coil. A drive shaft having a coil surrounded by a tube, a coil bonded to an inner coaxial cable, and an inner and outer coil that interfere with one another are shown. A superelastic coil is formed by continuously winding a wire about a mandrel, heating a region of the wire to render it superelastic, and after cooling, removing it from the mandrel.

BACKGROUND OF THE INVENTION

This invention relates to drive shafts used in acoustic imagingcatheters.

Acoustic imaging catheters are used in medicine to visualize theinternal conditions of the body, such as the condition of the walls ofthe vascular system. The imaging catheters comprise a transducer probeattached to the end of a flexible rotating drive shaft. The drive shaftis used to insert the transducer into the body, and to rotate thetransducer at high speed to produce a 360 degree image.

One drive shaft that has been employed comprises two cross-wound,multifilar stainless steel interlocking coils, as described in Crowleyet al., U.S. Pat. No. 4,951,677, incorporated by reference.

It is important that the drive shaft be flexible enough to pass throughtortuous passages in the body. The drive shaft should also haveone-to-one rotational fidelity between its proximal and distal ends toavoid image smearing. Acoustic imaging is made more powerful when thedrive shaft, and consequently the catheter itself, has a very smallouter diameter, enabling it to penetrate into more restrictive regionsof the body.

SUMMARY OF THE INVENTION

In acoustic imaging catheters there has been difficulty in achievingtrue one-to-one fidelity between the rotation of the driver at theproximal end of the catheter, and the rotation of the transducer at thedistal end. Lack of such fidelity produces artifacts in the image thatimpairs its quality and usefulness. To address this problem, attentionin the past has been paid to maximizing the torsional rigidity of theflexible drive shaft employed.

We have discovered that lack of fidelity can be addressed in another,highly effective way, by employing a drive coil fabricated at least inpart of relatively low modulus of elasticity metal, e.g. having elasticdeformation in the range of about 3% to 9%, preferably employingsuperelastic metal with recoverable deformation in the range of 4% to7%. We have realized that the relative kink-resistance of a coil of suchmetal prevents the formation of micro-kinks and other disturbances inthe geometry of the drive coil, such that, during rotation, the coilrotates much more smoothly when bent into the curves that occur in thenatural ducts of a patient, e.g. the aortic arch. With such a coil werealize that the rotational velocity of the distal tip will match veryclosely the rotational velocity of the proximal driver.

According to one aspect of the invention, a flexible, rotatable shaft isprovided, comprised of inner and outer tubular members in mutuallyinterfering contact along their length, at least one of the tubularmembers comprising a wire, at least a portion of the wire being exposedto forces tending to produce kinks, the portion being comprised of asuperelastic alloy, whereby substantial mechanical fidelity is achievedand resistance to damage improved by virtue of the superelasticity ofthe portion of the rotatable shaft.

According to another aspect of the invention, an ultrasound imagingcatheter is provided, comprising a hollow catheter shaft, an ultrasoundtransducer located distally from the proximal end of the hollow shaft,and a drive member extending from a proximal drive mechanism, throughthe hollow shaft to the transducer and being rotatable within the hollowcatheter and in rotatable drive relationship with the transducer, thedrive member comprising an elongated rotatable shaft comprised of atleast one tightly wound coil of metal having characteristic elasticdeformation under stress in the range of about 3% to 9% and a restraintmeans associated with the coil over the length of the coil effective toprevent torsional deflection of the coil. Preferably in this ultrasoundimaging catheter, the metal is superelastic.

In another preferred embodiment, a torque transmitting elongatedassembly, e.g. a torquable catheter, useful for introduction through aduct of the body comprises an elongated rotatable shaft of at least onetightly wound coil of metal. The metal has a characteristic elasticdeformation under stress in the range of about 3% to 9%. The assemblyalso has a restraint means over the length of the coil to preventtorsional deflection of the coil.

In the presently most preferred embodiment, the drive shaft or thetorque transmitting assembly comprises inner and outer, closely woundmultifilar coils, the coils being wound in opposite directions, eachfabricated of superelastic metal, the coils held together in interferingrelationship such that they mutually resist unwinding in response totorque or change in torque conditions. Preferably, during manufacture,the inner coil, after fabrication and heat treating to render itsuperelastic, is wound down on a smaller mandrel with elasticdeformation, thus achieving a smaller diameter before insertion into theouter coil, so that upon release it will spring to a larger diameter toachieve at least an original level of interference with the outer coil.In preferred embodiments, the multifilar construction is comprise ofbetween 3 and 10 filaments.

Other embodiments, however, are within the broader aspects of theinvention.

A drive coil of superelastic metal is advantageously combined withanother elongated device that provides resistance to winding orunwinding of the coil. Among such embodiments is a dual coil assembly,in which only the outer coil is superelastic; because the outer coil isexposed to the greater kink-producing stresses, such a combinationoffers advantages of the invention, while being easy to assemble. Theinner coil, formed e.g. of stainless steel, can be held in its originalcoiled state during insertion, but when released during assembly, willtend naturally to spring to a larger, interfering diameter with theouter superelastic coil, without need for special steps.

Another preferred embodiment comprises the combination of a closelywound multifilar coil of superelastic metal, about which a thinstretch-resistant sleeve of stiff polymeric material is closely fit, orabout which such a tube is heat-shrunk. The resistance to kinking isagain achieved by the superelastic coil, while the sleeve resists anytendency for the coil to unwind. As the wall thickness of such a sleevecan be of the order of 0.0002 inch (0.005 mm) the entire assembly can bequite small, capable of accessing very restricted regions of the body.In some embodiments, the coil is expanded into interfering contact withthe surrounding sleeve.

In still another preferred embodiment, there is a tightly woundmultifilar coil of superelastic metal, through which a co-axial cable isthreaded, for communication to and from the distal transducer from theproximal end, the coaxial cable in this case serving as anunwind-resistant member, the coil being bonded along its length to thesurrounded coaxial cable.

In another preferred embodiment of the invention, a coil of superelasticmaterial is formed from a running length of wire with latentsuperelastic properties from a supply. The wire is wound continuouslyabout a mandrel before being heated to stress relieve the wire andrender it superelastic. After the coil is formed, it is removed from themandrel.

In preferred embodiments, winding is performed by a pair of windingpoints, and in other embodiments the winding is performed by a rotatingdie. In preferred embodiments, the heating and winding are performedwhile the length of wire is in an inert gas chamber.

Nitinol coils manufactured in the various ways described above can bejoined to or embedded in the walls of thin tubes, or coated to form suchtubes, to enhance the compression resistance of thin walled tubularmembers over a wide range of tube diameters for use in catheters. Theprovision of such kink resistant catheter walls is another importantfeature of the present invention.

In various preferred embodiments, the wire of the coil is of circularcross section; in other embodiments the wire is of oval cross section,and in other embodiments the wire is of rectangular cross section.Preferably, the wire of which the coil is comprised is of radialdimension between about 0.012 and 0.001 inch depth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing introduction into the body of anacoustic imaging catheter according to the invention, while FIG. 2illustrates severe bending of the catheter as it accesses the heart.

FIG. 3 is a longitudinal, partially cut-away view of the distal end ofthe catheter.

FIG. 4 is a stress-strain curve for superelastic nitinol.

FIGS. 5 and 6 are longitudinal views of a partially constructed driveshaft.

FIGS. 7, 8 and 9 are schematic diagrams of a nitinol coil underconstruction.

FIG. 10 is a cut-away view of a drive shaft in one embodiment.

FIGS. 11 and 12 are side and cross-sectional views of the drive shaft inanother embodiment.

FIG. 13 is a cut-away view of a drive shaft.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

General Structure

Referring to FIGS. 1 and 2, a micro acoustic imaging catheter 10 imagesthe body with a miniature rotatable transducer 12 in its distal end 13which is positioned in the body, e.g. in a blood vessel 14 or the heart15. The transducer is driven by a hollow drive shaft 16 placed within acatheter sheath 18. A coaxial cable inside the drive shaft connects thetransducer to a relatively rigid connector 20 joining the catheter to acontrol system 22. The control system moves the catheter and monitorsand displays the returned transducer signal.

In an ultrasound imaging system, the relative position of the ultrasoundtransducer must be accurately known at all times to avoid imagedistortion of the return signal at the controller. Since the positioninformation is measured from the proximal end 24 of the drive shaft inthe preferred embodiment, it is important to have a one-to-onetransmission of motion with complete fidelity, meaning that a rotationof the proximal end of the drive shaft at a constant speed causes thetransducer to rotate at a corresponding constant speed.

We have realized that even a high fidelity drive shaft may not have aconstant rotation speed at the distal end due to miniature kinks andother permanent small distortions that may arise, for instance due tobeing coiled on the shelf for a period or subject to mishandling. Blurin the image due to lack of one-to-one fidelity is termed drive shaftartifact.

Structure of the drive shaft

By forming a drive shaft coil, in at least its critical region, from ametal whose characteristics permit a substantial amount of recoverablestrain, e.g. superelastic nitinol material, we realize that thesedetrimental microkinks can be avoided. Referring to FIG. 3, the driveshaft 16 in one embodiment comprises an inner coil 40 and outer coil 42of wound nitinol. The outer coil has an outer diameter 0.03 and an innerdiameter d_(i) of 0.017" and the inner coil has an inner diameter D_(i)of approximately 0.010" and an outer diameter D_(O) of 0.015". Each coilis of multifilar construction, having between 3 to 10 filaments, one ofwhich (44) is shown by shading, each made of a wire with a minimumcross-sectional diameter of about 0.002". In other embodiments, theouter diameter of the drive shaft as a whole ranges from about 0.012" toabout 0.06", with wire diameters ranging from about 0.002" to about0.007", respectively.

The coils are closely wound, in counterwound relationship, with an innerpitch angle α₀ and α₁ where α₀ is smaller than α₁, e.g., 22.5 and 31degrees, respectively. The pitch angles are chosen to eliminate space 46between turns of the wires, and to apply a substantial part of thestress from either tension or compression in the direction along theaxis of the wire filaments. The two coils fit together, as describedbelow, so that they interfere with one another when rotated in a givendirection, i.e. the outer coil will tend to contract while the innercoil tends to expand, each thereby resisting the radial change of theother. The interference significantly increases torsional stiffness inthe rotational direction, resulting in a high fidelity drive shaft.

In this preferred embodiment, each coil in the drive shaft is made of anitinol alloy, having an ultimate tensile strength of 250,000 to 300,000psi when drawn. The alloy is available from Furukawa Electric Companylocated in both Japan and California. After winding, the coils are heattreated to render them superelastic.

During use, the nitinol alloy exhibits superelastic characteristicsunder stress, i.e. it undergoes reversible deformation, changing fromAustenite to stress-induced Martensite, as shown in FIG. 4. Whenoptimized for superelasticity at body temperature, the alloy has aloading plateau 50 of approximately 100,000 psi, and an unloadingplateau 52 of approximately 50,000 psi with a temperature transition(A_(f)) of around 0 to 5 degrees C.

In other embodiments, the wire is a nitinol alloy selected from a widerange comprising from about 40% to 60% nickel with the majority of thebalance being titanium. Nitinol alloys having a third element, forexample, chromium, vanadium or iron as a third element are generallystiffer and stronger than a pure nickel-titanium alloy. The preferredrange for the ultimate tensile strength of the nitinol alloy is from200,000 to 400,000 psi with the material exhibiting 3% to 9% reversibleelastic deformation; a particularly useful material has 275,000 psitensile strength with 7% elastic deformation. The lower range of elasticdeformation (3-4%) is provided by either cold worked, non-superelasticMartensite or Austenite nitinol alloy, or in some cases titanium alloys.

Alloys below the lower limit of elastic deformation (about 3%) though insome cases providing good torsional fidelity do not demonstrate thebeneficial properties achieved with the present invention. Alloys withelastic deformation above the upper bound, about 9%, permit detrimentalwindup in a coil that results in drive shaft artifact.

Coils that are small in diameter (e.g., 0.020" outer diameter or less)are in some cases made of alloys with higher tensile strengths, up to400,000 psi. A higher tensile strength is achieved by increasing thepercentage of nickel in the alloy.

The cross section of the nitinol wire in various embodiments iscircular, oval or rectangular, with a radial dimension (diameter in thecase of a circular cross section) ranging from 0.001" to 0.012". Acircular cross section provides the greatest coil flexibility andlargest coil wall thickness, whereas a rectangular cross-sectiondecreases the wall thickness at the expense of some loss of flexibility.A rectangular cross section is appropriate, for instance, when the needfor smallness of the drive shaft is the primary constraint and whenflexibility is relatively less important for example, when the driveshaft does not have to bend tightly but the catheter's outer diametermust be 0.020" or smaller. Such wire may be of strip form of thicknessabout 0.002". The stiffness of the rectangular wire is minimized bydecreasing the width of its cross section, i.e. in the direction ofbecoming square.

An oval cross section is seen to be a useful compromise between thesetwo forms in certain instances, having the advantage of lowering thewall thickness while providing significant flexibility.

Under certain circumstances it is conceived to use triangular crosssections to provide high flexibility in selected regions of the driveshaft. A triangular wire flexes uniformly and provides space betweeneach coil to flex into the other, instead of rolling over one another.

Manufacture of the drive shaft

The drive shaft is manufactured by first winding the selected number ofnitinol wire filaments into a coil about a mandrel and securing its freeend in tightly wound condition, preferably by means of adhesive, tape ora clamp. The tightly wound coil, while remaining on the mandrel, is thensubjected to an annealing temperature of 450 C. for fifteen minutes torender it superelastic.

In another embodiment, heating is progressive, only a small section ofwire being heated to 450 C. at any time as it is being continuouslywound around a mandrel. The continuous coil coming off the mandrel isthen wound on a drum.

If the outer coil of an interfering coil construction is first producedby the process described above, the next step is to wind the inner coiltightly on a mandrel, in a direction opposite to the direction ofwinding of the outer coil. The inner coil is then heat treatedidentically to the outer coil and removed from the mandrel. The innercoil is now tightened by sliding it onto a smaller mandrel, securing itat one end, and then either winding, preferably, or stretching the coil,until the coil's inner diameter conforms to that of the smaller mandrel.This can advantageously reduce the outer diameter of the inner coil byabout 0.01".

Referring to FIG. 5, the inner coil 40 wound in conventional manner onthe mandrel 60 is now small enough to be inserted in the outer coil 42without interference. After the inner coil is inserted in the outercoil, the inner coil is released so that it springs toward its originaldiameter, causing it to engage the inner diameter of the outer coil withinterfering contact. The mandrel is then removed and the two coils aresecured at one end together by an adhesive, such as a high-temperatureepoxy, or by a clamp, such as a copper or steel wire tightly woundaround the clamped end.

After the coils are assembled, the inner coil is torqued in the oppositedirection in which it was wound to expand it, while the outer coil issimultaneously torqued to reduce its diameter, causing the two coils tointerfere more tightly. Since the coils are counterwound, the torque isapplied in the same direction on each of the coils. This results in thebands of the multifilar elements being uniformly distributed.

In another embodiment, the inner coil 40 is released from a firstmandrel and then, for assembly, attached to a "fishing" line 70, shownin FIG. 6. The inner diameter of the inner coil is reduced by applyingtension to the line while pulling the inner coil through the outer coil42.

In another production method, shown in FIG. 7, a nitinol wire 44 isdrawn from a supply roll 80 through feed rollers 82, and then passesthrough winding points 84, a pair of which bend the wire into a coil 86.The winding points are made of a very hard substance, such as steel,that is not subject to wear. A heating stage 88 heats the coil comingoff the winding points, making wire 44 superelastic. The coil thenslides onto a mandrel 90. After the superelastic coil unwinds off themandrel, it springs back to its initial shape before being wound on atake-up reel 92.

In another method of manufacture of the coil, shown in FIG. 8, a wire 44winds off a supply roll 80 to feed rollers 82. The wire is then pulledonto a rotating die 100 in the form of a screw with a central mandrel. Aforming heater 88 heats the wire wound onto the mandrel as the dierotates. Mandrel 90 holds wound coil 86 as it cools, and a take-up reel92 takes up the coil as it winds off mandrel 90.

The rotating die is placed in an inert gas chamber 110, seen in FIG. 9,to help prevent contamination of the nitinol. Multiple spools 92 windthe multiple wires 44 with a winding machine that has a variable driverate according the number of filaments being wound or the band width ofthe multifilar set of filaments. The number of spools used are from 3 to8. The wires are led to a winding head, wrapped around a mandrel, andtaped or mechanically secured. As the mandrel rotates, the wire isdeformed under a wear resistant carbide shoe and wrapped around themandrel. The carbide shoe maintains tension on the non-superelastic wireto produce tightness of the coils. When the coil on the mandrel isheated, the heat treating relaxes tension in the wire and the resultingsuperelastic coil is stress-neutral.

We have found that a drive shaft made from the superelastic coils ismore resistant to mishandling damage, such as being twisted or bentduring manufacture as well as when the catheter may be caught betweenmedical devices in a lab or during insertion. Kinks are much less likelyto form in the drive shaft, so that the resulting image will be free ofdrive shaft artifact.

Final assembly of the imaging catheter

Returning to FIG. 3, after the drive shaft is assembled, a very small(0.0055" to 0.009" outer diameter) electrical coaxial cable 120 isplaced through its center. A transducer housing containing a transduceris attached to one end of the drive shaft with an epoxy. The transduceris connected to the coaxial cable 120, and the drive shaft is connectedto an electrical connector. Such catheters are made in 6.0 French, 4.8French, 3.5 French, and 3 French sizes.

For some applications, the preferred outer dimension of the catheter is0.018" with a 0.013-0.014" outer diameter drive shaft if the catheterhas a sheath. This is accomplished with 0.002" diameter round crosssection wire, a 0.0055" outer diameter coaxial cable, and a 0.006"diameter mandrel.

Other embodiments of the drive shaft

In other embodiments, the drive shaft has a coil combined with aconstraining element that opposes any change in size of the coil as itrotates. If the constraint is placed inside the coil, the coil is drivenin the direction tending to reduce its diameter while the constraintresists contraction of the coil. If the constraint is outside the coil,the coil is driven in the direction tending to unwind the coil while theexterior constraint resists expansion of the coil. Such constructionenables high fidelity, flexible drive shafts to be formed with theadvantages of the present invention.

In the embodiment shown in FIG. 10, the drive shaft has an inner coil115 of stainless steel and an outer coil 122 of nitinol. The steel coilhas an inner diameter d_(i) of 0.008" and an outer diameter d_(o) of0.012"; the nitinol has an outer diameter D_(o) of 0.016".

In this arrangement, the outer superelastic coil, which, due togeometrical considerations, is stressed more during bending than is theinner coil, is better able to resist yielding or kinking than a steelcoil. The springback properties of the steel coil are used to easilyachieve good distribution of the coils and an initial level ofinterference between the inner and outer coil, eliminating the need towind the inner coil on a reduced size mandrel before insertion into theouter coil.

A structure with an inner superelastic coil and an outer stainless steelcoil can provide good torsional performance while also providingadequate resistance to kinking under certain circumstances.

Referring now to FIG. 11, the drive shaft in another embodiment iscomposed of a single multifilar nitinol coil 130 and an outside sleeve132 of a very thin (0.002") polyester material that constrains the coil.The polyester sleeve has the lateral support of the nitinol all alongits length so that under axial columnar load it does not kink, i.e., itis not subject to columnar collapse.

Use of the thin polyester outer sleeve enables the total coil wallthickness of the drive shaft to be quite small. The imaging catheterusing this type of drive shaft therefore can have an extremely smallouter diameter and be capable of accessing very restricted regions ofthe body.

The nitinol coil and the polyester tube are constructed by winding theinner nitinol coil on a reduced size mandrel as mentioned above, fixinga tube around the coil and mandrel, and releasing tension on the coil sothat it springs out against the tube. In another embodiment, the driveshaft is made by placing the inner coil on a mandrel, and then heatshrinking over that inner coil a heat shrinkable polyester tube.

In another embodiment, seen in FIG. 12, the nitinol coil 130 is embeddedbetween two polyester coatings 132, 134. The layered construction has aninner coil that is a braided wire instead of a spring winding, that iswound over an existing length of extruded tubing. Polymer tubings aresubsequently shrunk over the braided winding. The final wall thicknessof the drive shaft is 3 to 4 layers thick.

In yet another embodiment, instead of using a preformed tube, the coilis run through an extruder that extrudes the polyester layer on theoutside of the coil. The outer diameter of the coil and the gap betweeneach winding of the coil is tightly controlled by winding the coil on amandrel, allowing a very uniform coating of polymer to be placed on thecoil to form a flexible drive shaft. Such a drive shaft can haveapplications in imaging catheters with an outer diameter of 0.018 inchesor smaller.

Nitinol coils manufactured in the ways just described can thus be joinedto or embedded in the walls of thin tubes, or coated to form such tubes,to enhance the compression resistance of thin walled tubular membersover a wide range of tube diameters for use in catheters. The provisionof such kink resistant catheter walls is another important feature ofthe present invention.

Referring now to another embodiment of a drive shaft for acousticimaging, shown in FIG. 13, a single nitinol coil 140 is bondedperiodically to a coaxial cable 120 in its center to maintain stress onthe coil. The coaxial cable is an electrical transmission line with0.0055" to 0.010" outer diameter. The coaxial cable may also have anouter sheath that is plastic. If it is unsheathed, the coaxial cable hasan inner conductor with an insulator that is very carefully controlledin thickness and an outer conductor (copper) which conforms to theinsulator.

After the coaxial cable is threaded through the nitinol coil, anadhesive is applied to bind the coil to the coaxial cable along itsentire length with half an inch to two inches spacing between successivebonds 142. The adhesive is a flexible epoxy in order to have a flexibledrive shaft, and is low viscosity, so that it can penetrate easilybetween the windings of the coil. The coil is radially compressed duringthe bonding to reduce the space between the coaxial cable and the coiland to maintain the coil in a state of tension. Bonding the coil to thecoaxial cable thus can limit the amount of wind up that may occur overthe length of the coil, and thus can serve to reduce drive shaftartifact.

Using a single layer coil bonded to a coaxial cable or to polyestertubing is a preferred embodiment in certain instances when the wallthickness of the drive shaft needs to be limited to achieve a smallouter diameter. The single layer coil construction reduces the outerdiameter of the drive shaft by two wire diameters over an interferingdual coil construction. This enables the drive shaft to enter deeplyinto the body and into highly restricted regions, such as the coronaryarteries and the neurovascular system. Larger coils, such as the dualcoil drive shaft described above, are advantageous in imaging thegastrointestinal, urinary and esophageal tracts, the gall bladder, theperipheral arteries and other body ducts.

Other embodiments are within the following claims.

What is claimed is:
 1. A flexible, rotatable shaft comprised of innerand outer tubular members in mutually interfering contact along theirlength, at least one of said tubular members comprising a wire, at leasta portion of said wire being exposed to forces tending to produce kinks,said portion being comprised of a superelastic alloy, wherebysubstantial mechanical fidelity is achieved and resistance to damageimproved by virtue of the superelasticity of said portion of saidrotatable shaft.
 2. An ultrasound imaging catheter comprising a drivemember extending from a proximal drive mechanism, and an ultrasoundtransducer coupled in rotatable drive relationship to the distal end ofsaid drive member, said drive member comprising an elongated,torque-transmitting, flexible, rotatable shaft comprised of at least onecoil and a restrainer associated with said coil over the length of saidcoil, said restrainer being constructed and arranged to radiallyconstrain said coil, said coil that is so constrained being comprised ofmetal that has the property that it can undergo strain in the range ofabout 3% to 9% without exceeding its elastic limit, said propertyimparting resistance of said coil to kinks when said coil is exposed toexternally applied strain inducing forces, the resultant kink resistanceof said coil of said shaft serving to maintain mechanical fidelity ofsaid shaft after exposure to said strain inducing forces.
 3. Theultrasound imaging catheter of claim 2 wherein said metal issuperelastic.
 4. The ultrasound imaging catheter of claim 3 wherein saidrestrainer comprises a second tightly wound coil of superelastic metal,said coils being wound in opposite directions, and one of said coilsdisposed coaxially within the other in an interfering relationship. 5.The ultrasound imaging catheter of claim 3 wherein said superelasticcoil has fitted therein a counterwound stainless steel coil that issprung into internal interfering contact with said superelastic coil. 6.The ultrasound imaging catheter of claim 2 wherein said coil is disposedwithin a tightly surrounding sleeve of stiff polymeric material.
 7. Theultrasound imaging catheter of claim 6 wherein said sleeve is in aheat-shrunken state about said coil.
 8. The ultrasound imaging catheterof claim 6 wherein said coil is expanded into interfering contact withsaid sleeve.
 9. The ultrasound imaging catheter of claim 2 wherein acoaxial electrical cable extends through said coil for transmittingsignals between said transducer and a proximal signal receiving andsending device, and said coil is adhesively secured to said cable alongthe mutual length thereof in manner whereby said coaxial cable is in astress transferring relationship to said coil, serving as saidrestrainer to resist unwinding of said coil.
 10. The ultrasound imagingcatheter of claim 2 wherein said restrainer comprises an adhesivelysecured layer of stress resistant thermoplastic polymer.
 11. Theultrasound imaging catheter of claim 2 wherein said coil is ofmultifilar construction comprised of between 3 and 10 filaments.
 12. Theultrasound imaging catheter of claim 2 wherein the wire of which saidcoil is comprised is of circular cross-section.
 13. The ultrasoundimaging catheter of claim 2 wherein the wire of which said coil iscomprised is of oval cross-section.
 14. The ultrasound imaging catheterof claim 2 wherein the wire of which said coil is comprised is ofrectangular cross-section.
 15. The ultrasound imaging catheter of claim2 wherein the wire of which said coil is comprised is of radialdimension between about 0.012 and 0.001 inch depth.
 16. Atorque-transmitting flexible assembly useful for introduction through aduct of the body comprising an elongated rotatable shaft comprised of atleast one coil and a restrainer associated with said coil over thelength of said coil, said restrainer being constructed and arranged toradially constrain said coil, said coil that is so constrained beingcomprised of metal that has the property that it can undergo strain inthe range of about 3% to 9% without exceeding its elastic limit, saidproperty imparting resistance of said coil to kinks when said coil isexposed to externally applied strain inducing forces, the resultant kinkresistance of said coil of said shaft serving to maintain mechanicalfidelity of said shaft after exposure to said strain inducing forces.17. The assembly of claim 16 where said metal is superelastic.
 18. Theassembly of claim 17 wherein said restrainer comprises a second tightlywound coil of superelastic metal, said coils being wound in oppositedirections, and one of said coils disposed coaxially within the other inan interfering relationship.
 19. The assembly of claim 17 wherein saidsuperelastic coil has fitted therein a counterwound stainless steel coilthat is sprung into internal interfering contact with said superelasticcoil.
 20. The assembly of claim 16 wherein said coil is disposed withina tightly surrounding sleeve of stiff polymeric material.
 21. Theassembly of claim 20 wherein said sleeve is in a heat-shrunken stateabout said coil.
 22. The assembly of claim 20 wherein said coil isexpanded into interfering contact with said sleeve.
 23. The assembly ofclaim 16 wherein an electric cable extends through said coil fortransmitting signals, and said coil is adhesively secured to said cablealong the mutual length thereof in manner whereby said cable is in astress transferring relationship to said coil, serving as saidrestrainer to resist unwinding of said coil.
 24. The assembly of claim16 wherein said restrainer comprises an adhesively secured layer ofstress resistant thermoplastic polymer.
 25. The assembly of claim 16wherein said coil is of multifilar construction comprised of between 3and 10 filaments.
 26. The assembly of claim 16 wherein the wire of whichsaid coil is comprised is of circular cross-section.
 27. The assembly ofclaim 16 wherein the wire of which said coil is comprised is of ovalcross-section.
 28. The assembly of claim 16 wherein the wire of whichsaid coil is comprised is of rectangular cross-section.
 29. The assemblyof claim 16 wherein the wire of which said coil is comprised is ofradial dimension between about 0.012 and 0.001 inch depth.
 30. Atorque-transmitting, flexible, rotatable shaft comprised of inner andouter elongated members in mutually interfering contact along theirlengths, at least one of said members comprising a coil, and the otherof said members being constructed an arranged to radially constrain saidcoil to enable transmission of torque through said coil, at least aportion of said coil that is so constrained being comprised of asuperelastic alloy, said alloy serving to impart resistance of saidportion to kinks when said portion is exposed to externally appliedstrain-inducing forces, the resultant kink resistance of said coil ofsaid shaft serving to maintain mechanical fidelity of said shaft afterexposure of said shaft to said strain-inducing forces.